International projectionist (July-Dec 1934)

INTERNATIONAL PROJECTIONIST October 1934 BRUSH HOLDER Showing proper brush fitting. There is a slight amount of play between the brush and its holder, so that it will not stick. The arrow within the curved line shows the direction of rotation of the armature, the curved line representing the commutator. A X\ X The brush touches the lower part of the holder at the right, while at the upper end it touches the holder at the left. X represents the strip of sandpaper. When the sandpaper is drawn in the direction shown by the arrow above it, the brush is pulled into the same position as it occupies during normal operation of the machine, and this fit will be right. If the paper be drawn the other way, as shown at B, the brush is tipped the opposite way in the holder, consequently changing the angle it makes with the commutator. If the brush would stay in this position when in normal operation, everything would be in order; but when the machine starts, the brush assumes the position as at A, with the result that the face of the brush does not fit the commutator. This is shown at C. Not only will this cause overloading of the brush, as a small area has to carry the current that the entire brush is supposed to carry, but it will cause sparking and improper commutation. — A. C. Schroeder. NOT TOUCHING AT / CORNER ' tween the commutator or ring and the brush. A medium voltage drop for a positive plus negative brush is 1.2 and 1.8 volts. Abrasiveness is the degree of surface wear of commutators and rings. Commutators with hard mica, not undercut, and machines with poor commutation require abrasive brushes. Bright polish on a commutator is an indication of an abrasive action, while dark gloss indicates good commutation. Non-abrasive brushes should be used with undercut commutators. Brush pressure is given in pounds per square inch of contact surface. Tension or brush pressure, is an extremely important factor in the life of a brush. In general it is best to use the lowest pressure that will give good contact, but never less than 1% lbs. per square inch, except with some grades of graphite brushes. When large metallic brushes are used, more spring tension must be applied to the brushes on the underside in order to compensate for the weight of the brushes. The bevel of a brush is the angle included between the bevel face of the brush and the plane at right angles to the length of the brush. Bevels vary by steps of 5 degrees. The length of a beveled brush is the length of the square ended brush from which the beveled brush was made. "End" bevels on a brush are known as "contact face bevel" and "spring end bevel." The running angle of a brush is its position on a commutator with reference to the direction of rotation and is classed as "trailing", "leading" and "radial". Shunts or "pigtails" are flexible copper leads securely fastened to the brushes. Failure of these shunts to carry the current causes brush holder springs to lose their temper and burn off. Brush holders become pitted and sparking may occur. Copper plating of brushes with shunts is for the purpose of lowering the voltage drop between the brush and the shunt and to maintain this condition. Length of a brush is the maximum overall dimension in the direction in which the brush feeds to the commutator or collector ring. Width is the dimension at right angles to the length and to the direction of rotation. Thickness of a brush is the dimension at right angles to the length in the direction of rotation. Flexibles, shunts or pigtails are attached to brushes (except those used on clamp-type holders), for the purpose of carrying most of the current passing through the brushes — preferably all of it. If a shunt was not used, the current would go through the holder and the finger spring. This would very likely cause arcing which would result in injury to both the brush as well as the spring. There are several different methods used for fastening shunts to brushes such as: bolted, screwed, soldered, spun rivets, cemented and with wedges inserted in holes drilled in the brushes. The separate type of pigtail is rarely used any more as the other methods are better. On some of the softer grades of carbon graphite and graphite brushes, metal clips are fastened over the top so as to prevent wear from the pressure fingers. PROJECTIONS By Frank Dudiak MANY important projection problems will be considered during the Fall Meeting of the S. M. P. E., outstanding among which is the question of leel length. Fortunately, the sessions will be held in New York City and are assured of a strong projectionist representation. In its report of last Spring the Projection Practice Committee of the Society went on record as being absolutely opposed to the proposal of standardizing the 1,700-foot reel, as was suggested, for the reason that with a 1,700-foot maximum length the average length would be less than 1,500 feet, or possibly even 1,350 feet. Thus, said the Committee, the doubling of reels would continue as before. Modern projectors are equipped with magazines which can accommodate more than 3,000 feet of film, thus it is necessary that any single reel be of such length as to make impossible its doubling, or joining, with another reel. It would appear that the Committee is perfectly right in taking a determined stand in favor of the longer basic length. Publication recently of the membership roster of the S. M. P. E. reveals a steady growth in projectionist membership. Under the new rules, projectionists may become associate members of the Society for as little as $6 a year, or 50c a month. The Journal which is sent monthly to each member alone is worth this investment. One projectionist approached another with the following question: In the projector in question, approximately how many more revolutions is the reel in the upper magazine making per second when the diameter is six inches than it would if the diameter were ten inches? The answer to this little poser will appear in our next column. [HINT: The circumference of a circle is found by multiplying the diameter by 3.15, or the radius by 6.3.] Constant attention is necessary to minimize noise originating in the sound system. Dirty or corroded tube prongs are consistent troublemakers, causing a noise like rippling water or a loud crackling to issue from the horns. If the noise be caused by the tube prongs making poor contact, the trouble likely will be found in the most sensitive tubes in the system When the faulty tube is located, the prongs and tube socket should be cleaned with an ink or pencil eraser (not with sandpaper or other abrasive), after which they should be wiped with a cloth dampened with carbontetrachloride. When a man becomes chesty and orates on his knowledge of projection fi om A to Z, his listeners mentally interpret the range as being from A to B.